1. Filed of the Invention
The present invention relates to a compound prism consisting of a plurality of prisms having a reflective interface or reflective interfaces.
2. Description of Related Art
A compound prism is used, for example, as a beam splitter in an optical magnetic recording and reproducing device (optical disc device). The beam splitter comprises a unit prism with a triangular shaped cross section and a unit prism with a parallelogram shaped cross section which is adhered to the triangular unit prism. The laser beam is made incident upon the triangular unit prism and is transmitted through the parallelogram unit prism to be incident upon an optical disc. The light reflected by the optical disc is made incident again upon the parallelogram unit prism and is split into signal light and servo light by semitransparent films provided on the parallel surfaces thereof.
In such unit prisms, the surfaces are polished to form the reflective surfaces on which the semitransparent films are then evaporated. Surface polishing is effected to make the reflective surfaces as geometrically flat as possible, although the polished surfaces tend to be either somewhat convex or concave as a result of the tendency of polishing machines to have the characteristic of either convex or concave polishing. Note that "concave" and "convex" surfaces referred to herein mean concavity and convexity of the order of a laser beam wavelength. This slight concavity or convexity can not be completely removed, even by the precision polishing.
Assuming that the parallel surfaces of a parallelogram unit prism are, for example, concave, the inner surfaces (internal reflection surfaces) thereof by which the laser beam is reflected are convex surfaces, i.e., the laser beam is reflected by the convex inner surfaces. In particular, a convex wave front aberration in the direction of transmission is produced in the laser beam by the first reflection, and the convex wave front aberration is enhanced by the second reflection. Consequently, if the laser beam having the convex wave front aberration is made incident upon a servo system or a signal reproducing system of the optical disc device, the servo control and the signal reproduction will not be precise due to the convex wave front aberration.